Recently, there is an increasing demand for low fuel consumption of cars, and a demand for a tire assuring a low rolling resistance is also increasing. Accordingly, there is used a tire giving rolling resistance reduced by decreasing tan δ of rubber compositions constituting each member of a tire, not excepting a rubber composition for insulation.
Insulation of a tire is a member to be provided between a case and a component such as a tie gum provided between an inner liner and a case and an inner layer sidewall provided between a case and an outer layer sidewall, and is used for the purpose of improving durability of a tire mainly by reinforcing a case. Therefore, a proper rubber strength and E* (complex elastic modulus) are required for rubber compositions for insulation. Here, in order to secure a proper complex elastic modulus and low tan δ, reversion property at vulcanization is important.
Since a tie gum is located between an inner liner and a case, crack growth property and resistance to heat deterioration as in a rubber composition for an inner liner are not required particularly, and crack growth property, tear strength and cord adhesion as in a rubber composition for case topping are not required particularly. Moreover, since an inner layer sidewall is provided between a case and an outer layer sidewall, crack growth property, tear strength and cut resistance as in a rubber composition for an outer layer sidewall are not required, and crack growth property, tear strength and cord adhesion as in a rubber composition for case topping are not required particularly.
A rubber composition for case topping has been used as a rubber composition for insulation so far. Rolling resistance of a tire can be reduced by using a rubber composition for case topping giving reduced rolling resistance as a rubber composition for insulation. However, as mentioned above, while proper rubber strength and reversion property are required for a rubber composition for insulation, crack growth property, tear strength and cord adhesion are also required for a rubber composition for case topping in addition to proper rubber strength and reversion property. Namely, it is possible to use a rubber composition for case topping as one for a rubber composition for insulation, but there is a problem that cost is increased.
Patent Document 1 describes an invention for improving rolling resistance and steering stability by using a rubber composition for sidewall, clinch and/or insulation comprising at least one vulcanization accelerator selected from the group consisting of a filler, sulfur, a citraconimide compound, an organic thiosulfate compound, an alkylphenol-sulfur chloride condensate and a specific metal salt of methacrylic acid in a specific amount based on a specific rubber component. However, since this rubber composition is not one for specific use for insulation such as a tie gum and an inner layer sidewall, cost is high and sheet processability is not enough for a rubber composition for insulation. Further, this rubber composition has improved cord adhesion and crack growth property, but these are over performance for application to a rubber composition for insulation.
Patent Document 2 describes an invention for enhancing moisture barrier property to improve durability of a tire by using a rubber composition for insulation comprising 5 to 70 parts by mass of a pulverized bituminous coal powder having an average particle size of not more than 0.1 mm based on 100 parts by mass of a diene rubber comprising 70 to 100 parts by mass of a natural rubber. However, improvement of rolling resistance is not taken into consideration.